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: The [[Hydroxide Ion (Chemistry)|Hydroxide ion]]s in an [[alkali]] will donate [[electron]]s to [[Hydrogen Ion (Chemistry)|Hydrogen ions]] to form [[water]]. This is a [[neutralisation]] [[Chemical Reaction|reaction]].
: The strength of an [[alkali]] is measured on the [[pH]] scale. The more [[Hydroxide Ion (Chemistry)|Hydroxide ions]] in a [[solution]] the lower the [[pH]].
===Examples===
[[Metal Hydroxide]]s form '''Hydroxide ions''' in [[water]].
: NaOH(aq) → Na<sup>+</sup>(aq)+OH<sup>-</sup>(aq)
: KOH(aq) → K<sup>+</sup>(aq)+OH<sup>-</sup>(aq)
: Mg(OH)<sub>2</sub>(aq) → Mg<sup>+2</sup>(aq)+2OH<sup>-</sup>(aq)
: Ca(OH)<sub>2</sub>(aq) → Ca<sup>+2</sup>(aq)+2OH<sup>-</sup>(aq)
: Al(OH)<sub>3</sub>(aq) → Al<sup>+3</sup>(aq)+3OH<sup>-</sup>(aq)
[[Metal Oxide]]s [[Chemical Reaction|react]] with [[water]] to form [[Metal Hydroxides]].
: Na<sub>2</sub>O(s) + H<sub>2</sub>O(l) → 2Na<sup>+</sup>(aq)+2OH<sup>-</sup>(aq)
: K<sub>2</sub>O(s) + H<sub>2</sub>O(l) → 2K<sup>+</sup>(aq)+2OH<sup>-</sup>(aq)
: MgO(s) + 2H<sub>2</sub>O(l) → Mg<sup>+2</sup>(aq)+2OH<sup>-</sup>(aq)
: CaO(s) + 2H<sub>2</sub>O(l) → Ca<sup>+2</sup>(aq)+2OH<sup>-</sup>(aq)
: Al<sub>2</sub>O<sub>3</sub>(s) + 3H<sub>2</sub>O(l) → 2Al<sup>+3</sup>(aq)+6OH<sup>-</sup>(aq)
[[Ammonia]] also [[Chemical Reaction|reacts]] with [[water]] to produce '''hydroxide ions''':
: NH<sub>3</sub>O(aq) + H<sub>2</sub>O(l) → NH<sub>4</sub><sup>+</sup>(aq)+OH<sup>-</sup>(aq)